WASHINGTON, June 17 (Reuters) - Both Iran and the U.S. can walk away from the memorandum of understanding they are set to sign on Friday, and upcoming talks are likely to focus on the precise sequencing of the steps previewed in the preliminary accord, a senior U.S. official told reporters on Wednesday.

Speaking on condition of anonymity, the official read out the 14-point memorandum that is due to be formally signed in Switzerland. They said the upcoming meeting there will be "critical" for ensuring that the memorandum of understanding can evolve into a comprehensive agreement.